From 7c2adb1d5c1f0b05dc030365f9a811a6431af0e1 Mon Sep 17 00:00:00 2001 From: Damien Diederen Date: Mon, 31 May 2021 11:21:23 +0200 Subject: [PATCH] nixos/lib/make-ext4-fs: Fix: `resize2fs -M' can leave insufficient slack The root filesystem resizing step, `resize2fs -M', does not provide any control over the amount of slack left in the result. It can produce an arbitrarily tight fit, depending on how well the payload aligns with ext4 data structures. This is problematic, as NixOS must create a few files and directories during its first boot, before the root is enlarged to match the size of the containing SD card. An overly tight fit can cause failures in the first stage: mkdir: can't create directory '/mnt-root/proc': No space left on device or in the second stage: install: cannot create directory '/var': No space left on device A previous version of `make-ext4-fs' (before PR #79368) was explicitly "reserving" 16 MiB of free space in the final filesystem. Manually calculating the size of an ext4 filesystem is a perilous endeavor, however, and the method it employed was apparently unreliable. Reverting is consequently not a good option. A solution would be to create some sort of "balloon" occupying inodes and blocks in the image prior to invoking `resize2fs -M', and to remove these temporary files/directories before the compression step. This changeset takes the simpler approach of simply dropping the resizing step. Note that this does *not* result in a larger image in general, as the current procedure does not truncate the `.img' file anyway. In fact, it has been observed to yield *smaller* compressed images---probably because of some "noise" left after resizing. E.g., before-vs-after: -r--r--r-- 2 root root 607M 1. Jan 1970 nixos-sd-image-21.11pre-git-x86_64-linux.img.zst -r--r--r-- 2 root root 606M 1. Jan 1970 nixos-sd-image-21.11pre-git-x86_64-linux.img.zst --- nixos/lib/make-ext4-fs.nix | 8 +++----- 1 file changed, 3 insertions(+), 5 deletions(-) diff --git a/nixos/lib/make-ext4-fs.nix b/nixos/lib/make-ext4-fs.nix index 33dbc8f5ec4..416beeb32f2 100644 --- a/nixos/lib/make-ext4-fs.nix +++ b/nixos/lib/make-ext4-fs.nix @@ -74,11 +74,9 @@ pkgs.stdenv.mkDerivation { return 1 fi - echo "Resizing to minimum allowed size" - resize2fs -M $img - - # And a final fsck, because of the previous truncating. - fsck.ext4 -n -f $img + # We may want to shrink the file system and resize the image to + # get rid of the unnecessary slack here--but see + # https://github.com/NixOS/nixpkgs/issues/125121 for caveats. if [ ${builtins.toString compressImage} ]; then echo "Compressing image"